The sphygmomanometric class of automated blood pressure monitors employs an inflatable cuff to exert controlled counter-pressure on the vasculature of a patient. One large class of such monitors, exemplified by that described in U.S. Pat. Nos. 4,349,034 and 4,360,029, both to Maynard Ramsey, III and commonly assigned herewith, employs the oscillometric methodology. In accordance with the Ramsey patents, an inflatable cuff is suitably located on the limb of a patient and is pumped up to a predetermined pressure above the systolic pressure. Then, the cuff pressure is reduced in predetermined decrements, and at each level, pressure fluctuations are monitored. The resultant signals typically consist of a DC voltage with a small superimposed variational component caused by arterial blood pressure pulsations (referred to herein as "oscillation complexes" or just simply "oscillations"). After suitable filtering to reject the DC component and to provide amplification, peak pulse amplitudes (PPA) above a given base-line are measured and stored. As the decrementing continues, the peak amplitudes will normally increase from a lower level to a relative maximum, and thereafter will decrease. These peak amplitudes together form an oscillometric envelope for the patient. The lowest cuff pressure at which the oscillations have a maximum value has been found to be representative of the mean arterial pressure ("MAP"). Systolic and diastolic pressures can be derived either as predetermined fractions of MAP, or by more sophisticated methods of direct processing of the oscillation complexes.
The step deflation technique as set forth in the Ramsey patents is the commercial standard of operation. A large percentage of clinically acceptable automated blood pressure monitors utilize the step deflation rationale. Accordingly, many subsequent developments have been directed at minimizing the duration of this step deflation period so as to minimize patient discomfort. For example, in U.S. Pat. No. 4,926,873 to Frankenreiter, the size of the deflation steps for a measurement cycle is varied from measurement to measurement as a function of the patient's actual blood pressure as measured in the preceding measuring cycle. This allows the duration of the measurement cycle to be minimized since extra steps can be avoided for patients with hypertension and more accurate measurements can be made for patients with hypotension. However, the duration of each deflation step within a particular measurement cycle is not varied.
On the other hand, in U.S. Pat. Nos. 4,543,962 to Medero et al., 4,889,133 to Nelson et al., and 4,949,710 to Dorsett et al., signal processing techniques are used to minimize the duration of each deflation step within a particular measurement cycle needed for detecting and processing the oscillation complexes. Such systems typically use a fixed "timeout" period at each pressure level to search for the oscillation complexes and only advance to the next step when one or more suitable oscillation complexes are detected-or the "timeout" is reached.
Unfortunately, the entire timeout period cannot be used to search for oscillation complexes because of a problem in step deflate/inflate oscillometric blood pressure monitors known as the "air effect." The "air effect" is a slow rise or fall in cuff pressure that results from a step inflate or step inflate. The air effect occurs when a quick pressure change occurs in the cuff and the pressure cannot stabilize immediately. The air effect has many different sources, such as the thermal changes in the air in the cuff as the air pressure is changed and the material properties of the cuff fabric. Generally, the air effect interferes with the small oscillations caused by the artery beneath the cuff by causing them to have modified pulse amplitudes. As a result, after a step inflate or deflate, the oscillations cannot be detected until the next heart cycle to allow time for the air effect to settle out. If the air effect is not given time to settle out, the oscillation amplitudes may be overestimated or underestimated, thereby causing errors in the blood pressure determination.
As the speed of analysis of the blood pressure envelope improves, larger deflate/inflate steps may be used to decrease the overall time for the blood pressure determination, thereby increasing the comfort to the patient. However, larger steps cause larger air effects which, in turn, cause longer determination times. As a result, the benefit of the larger steps are somewhat negated by the increased time needed at each step to detect the oscillation complexes.
It is, accordingly, a primary object of the present invention to minimize "air effects" and to shorten the time needed for air effects to settle out.
It is a further object of the present invention to prevent the underestimation or overestimation of the pulse amplitudes during an oscillometric blood pressure measurement.
It is yet another object of the present invention to minimize errors caused by "air effects" when large deflates are used between pulse measurements.